A known part comprises a composite body including a layer of ceramic fibers, for example glass fibers, the ceramic fibers being embedded in a thermoplastic polymeric matrix such as polypropylene.
This type of material is designated by the commercial term of “Sommold”.
In order to easily make this part, the usual method of the aforementioned type is known. In this method, a base mat comprising a mixture of ceramic fibers and polypropylene fibers is cut out beforehand to the form of the desired part.
Next, the mat is introduced between two heated plates which transmit the calories by conduction and by convection of the air trapped in the mat. This gives the possibility of rapidly and easily heating the base mat in order to cause melting of the polypropylene fibers. Next, the layer formed with the molten polymer is transferred into a cold mold which closes and compresses this layer down to the required thickness. The polypropylene flows through the glass fibers and forms, after cooling, the polymer matrix of the composite body which has the desired thickness, and the intended shape. The heating step before introduction into the mold simplifies the operation of the mold which does not have to be thermally regulated and reduces the cycle time, the preheating operation being performed in masked time during the molding operation per se.
The thereby produced composite body may be used as such, be assembled with a composite body of the same nature and form “double shell” parts or further be assembled on a honeycomb spacer in order to produce a composite part of the sandwich type. These parts receive in the majority of the cases, a cladding most often of the carpet type, either tufted or needled.
Generally, the mass proportion of glass fibers and of polypropylene fibers in the base mat is of the order of 50%.
The parts obtained by this method does not always give entire satisfaction. In certain applications, the requirements sheet for the part in particular implies resistance at a high temperature, being expressed by very low deflections under load, in particular at temperatures above 85° C. For example, this is the case for rear shelves of an automotive vehicle which are subject to temperatures of this order when the vehicle remains exposed to the sun for a long period of time.
Another drawback of Sommold parts lies in the fact that the technique for making the mat causes a certain number of glass fibers to be found oriented perpendicular to the plane of the mat. These poorly oriented glass fibers are again found even after molding within the finished part and may pierce the cladding thereby causing irritations for the operators and possibly for the final users.
In order to overcome this high temperature behavior problem of the composites based on ceramic fibers, the use, instead of polyolefin fibers, of a thermosetting polymer capable of crosslinking for generating three dimensional bridges between the macromolecular chains is known. Thermosetting polymers generally have high elastic moduli and a good high temperature behavior, with low creep. They are however difficult to recycle and the conformation of the thermosetting polymeric matrix may be complicated to apply industrially, essentially because of the duration of crosslinking which induces a high cycle time.
A thermosetting polymer family is very often associated with ceramic fibers and the family of unsaturated polyesters. Thus the term of GRP (glass-fiber reinforced plastic) in the majority of the cases makes reference to composites based on glass fibers reinforced with thermosetting unsaturated polyester resins. GRPs have been used for a long time as skins in sandwiched structures such as described in FR 2 580 991.
EP 0 628 406 proposes the use of different thermosetting resin types for producing skins of a structuring part of the sandwich type. Further, adhesion with the spacer is proposed so as to be achieved with resin which expands as a foam in order to partly penetrate into the spacer which allows reinforcement of the latter and ensures good adhesion with the skins.
Alternatively, EP-2 311 629 describes a structuring part of an automotive vehicle including a central spacer including a central spacer and two polymeric bodies attached on either side of the central spacer. Each body is obtained on the basis of two-component fibers in polyethylene terephthalate. In this case, the matrix as well as the reinforcement fibers are formed by a thermoplastic saturated polyester while the honeycomb spacer is also based on thermoplastic polyester. Because the spacer and the skins are of the same chemical nature, their adhesion may be strong which contributes to improving the flexural behavior. This product is easily recyclable since it is based on a single type of polymer.
Nevertheless, the method for applying these parts passes through a coating step which requires specific equipment and requires extrusion of two-component fibers which remains an expensive method. Moreover the elastic modulus of the terephthalate polyester fibers used as a reinforcement remains considerably less than that of the glass leading to composites of lesser performances.